The present invention pertains to improvements in the field of oriented multilayer polymeric films. More particularly, the invention relates to a substrate coated with an oriented multilayer polymeric film having improved flexibility and improved gas barrier properties, as well as to a method of forming such a film on a substrate.
When considering gas barrier properties of an extruded polymeric film coated on a substrate, it is already known in the art that the diffusion coefficient of any penetrating gas such as oxygen, carbon dioxide or water vapor through the polymer film decreases by increasing the crystallinity of the film. This can be achieved by a chemical approach (molecular design) and appropriate cooling rate (chilling in the coating process).
In the case of a substrate coated with an oriented multilayer polymeric film formed from a water-based polymer dispersion and wherein the polymer particles of each layer are oriented in the same direction, it is also known that a three-layer film provides a stronger barrier to gas than a two-layer film having the same weight, which in turn is much more efficient than a one-layer film also having the same weight. Water-based polymer dispersions comprise very small polymer particles having an average size ranging from 150 to 200 mm and containing macro-molecules. When coated on a substrate and properly dried to remove the water, a continuous film is formed.
Whereas scientists are still studying and modeling oriented multilayer polymeric films formed from water-based polymer dispersions, they all agree that these films have a weak flexibility compared to that of extruded polymeric films. This weak flexibility renders waterborne barrier coatings in the packaging industry less attractive. The film flexibility is weak mainly when the film is folded about a fold line parallel to the direction of orientation of the polymer particles in each layer of the multilayer film, causing the film to break at the fold line. This of course impairs the gas barrier properties of the film.
It is therefore an object of the present invention to overcome the above drawbacks and to provide a substrate coated with an oriented multilayer polymeric film which is formed from a polymer dispersion and which has improved flexibility as well as improved gas barrier properties.
It is another object of the invention to provide a method of forming the above film on a substrate.
According to one aspect of the present invention, there is thus provided a substrate coated with an oriented multilayer polymeric film, wherein the film comprises at least two layers of polymer particles oriented along two different directions with respect to one another.
Applicant has found quite unexpectedly that the presence of at least two layers of polymer particles oriented along two different directions with respect to one another in a multilayer polymeric film improves the flexibility of such a film as well as the gas barrier properties thereof.
The present invention also provides, in another aspect thereof, a method of forming the above oriented multilayer polymeric film on a substrate. The method according to the invention comprises the steps of:
a) conveying a substrate along a predetermined path at a predetermined travelling speed and in a predetermined travelling direction;
b) coating the substrate with a polymer dispersion containing polymer particles and a liquid dispersing medium to form on the substrate a first coating of the dispersion;
c) contacting the first coating with a first particle orienting roller driven for rotation about a first longitudinal axis thereof independently of the substrate so as to have a first tangential speed at a surface of the coated substrate, the first particle orienting roller having a first particle orienting pattern arranged at a first angle relative to the travelling direction of the substrate to cause orientation of the polymer particles of the first coating along a first predetermined direction;
d) drying the first coating to cause evaporation of the liquid dispersing medium and formation of a first layer of oriented polymer particles on the substrate; and
e) successively forming on the first layer at least one further layer of oriented polymer particles, each further layer being formed by:
i) coating a previously formed underlying layer of oriented polymer particles with the polymer dispersion to form on the underlying layer a further coating of the dispersion;
ii) contacting the further coating with a further particle orienting roller driven for rotation about a further longitudinal axis thereof independently of the substrate so as to have a further tangential speed at the surface of the coated substrate, the further particle orienting roller having a further particle orienting pattern arranged at a further angle relative to the travelling direction of the substrate to cause orientation of the polymer particles of the further coating along a further predetermined direction; and
iii) drying the further coating to cause evaporation of the liquid dispersing medium and formation of the further layer of oriented polymer particles on the underlying layer;
wherein at least one further angle is different from the first angle or at least one further tangential speed is different from the first tangential speed, thereby forming on the substrate an oriented multilayer polymeric film having at least two layers of polymer particles oriented along two different directions with respect to one another.
The polymer particles are preferably particles of a waterborne polymer. Examples of suitable waterborne polymers which may be used include polyvinylidene chloride, polyvinyl acetate, polyvinyl alcohol and styrene-butadiene copolymers. The liquid dispersing medium can comprise, for example, water, an alcohol or a mixture thereof.
According to a preferred embodiment of the invention, the first particle orienting roller comprises a first cylindrical member rotatable about the aforesaid first longitudinal axis and a first continuous helical land on the first cylindrical member over at least a portion of the length thereof, the first helical land forming a first continuous helical particle orienting groove along the first cylindrical member. The first land and the first groove define the aforesaid first particle orienting pattern. The first helical land may be defined by a single wire helically and tightly wound about a major portion of the length of the first cylindrical member.
According to another preferred embodiment of the invention, two further layers of oriented polymer particles are formed in step (e) by:
i) coating the first layer of oriented polymer particles with the polymer dispersion to form on the first layer a second coating of the dispersion;
ii) contacting the second coating with a second particle orienting roller driven for rotation about a second longitudinal axis thereof independently of the substrate so as to have a second tangential speed at the surface of the coated substrate, the second particle orienting roller having a second particle orienting pattern arranged at a second angle relative to the travelling direction of the substrate to cause orientation of the polymer particles of the second coating along a second predetermined direction;
iii) drying the second coating to cause evaporation of the liquid dispersing medium and formation of a second layer of oriented polymer particles on the first layer;
iv) coating the second layer of oriented polymer particles with the polymer dispersion to form on the second layer a third coating of the dispersion;
v) contacting the third coating with a third particle orienting roller driven for rotation about a third longitudinal axis thereof independently of the substrate so as to have a third tangential speed at the surface of the coated substrate, the third particle orienting roller having a third particle orienting pattern arranged at a third angle relative to the travelling direction of the substrate to cause orientation of the polymer particles of the third coating along a third predetermined direction; and
vi) drying the third coating to cause evaporation of the liquid dispersing medium and formation of a third layer of oriented polymer particles on the second layer.
The second angle is different from the aforementioned first angle or the second tangential speed is different from the aforementioned first tangential speed, whereby the second predetermined direction is different from the aforementioned first predetermined direction. The third angle is different from the second angle or the third tangential speed is different from the second tangential speed, whereby the third predetermined direction is different from the second predetermined direction.
Preferably, the second particle orienting roller comprises a second cylindrical member rotatable about the aforesaid second longitudinal axis and a first plurality of juxtaposed continuous helical lands on the second cylindrical member over at least a portion of the length thereof, the helical lands of the first plurality having a similar pitch and forming a first series of helical particle orienting grooves along the second cylindrical member, the lands of the first plurality and the grooves of the first series defining the second particle orienting pattern. The third particle orienting roller, on the other hand, comprises a third cylindrical member rotatable about the aforesaid third longitudinal axis and a second plurality of juxtaposed continuous helical lands on the third cylindrical member over at least a portion of the length thereof, the helical lands of the second plurality having a similar pitch and forming a second series of helical particle orienting grooves along the third cylindrical member, the lands of the second plurality and the grooves of the second series defining the third particle orienting pattern.
The aforementioned particle orienting roller provided with a series of helical particle orienting grooves is novel and constitutes a further aspect of the invention.
The present invention therefore provides, in a further aspect thereof, a particle orienting roller for orienting polymer particles present in a polymer dispersion coated on a substrate. The particle orienting roller according to the invention comprises a cylindrical member rotatable about a longitudinal axis thereof and a plurality of juxtaposed continuous helical lands on the cylindrical member over at least a portion of the length thereof. The helical lands have a similar pitch and form a series of helical particle orienting grooves along the cylindrical member for imparting a predetermined orientation to the polymer particles when the cylindrical member is rotated while being in contact with the polymer dispersion.
According to a preferred embodiment, the helical lands are defined by a plurality of juxtaposed wires helically wound about the cylindrical member, the helical particle orienting grooves being each defined between adjacent wires.
According to another preferred embodiment, the helical lands are defined by a plurality of helical ribs integrally formed on a peripheral surface of the cylindrical member, the helical particle orienting grooves being each defined between adjacent ribs.
According to a further preferred embodiment, the helical particle orienting grooves are integrally defined in a peripheral surface of the cylindrical member.
A particularly preferred oriented multilayer polymeric film formed on a substrate in accordance with the invention is an oriented three-layer polymeric film having a first layer comprising polymer particles oriented along a first direction, a second layer disposed on the first layer and comprising polymer particles oriented along a second direction angled at about 45xc2x0 relative to the first direction, and a third layer disposed on the second layer and comprising polymer particles oriented along a third direction parallel to the first direction.
As previously noted, the oriented multilayer polymeric film formed on a substrate in accordance with the present invention has improved flexibility and improved gas barrier properties.